Not many collectors are able to own an original Spiller & Burr, but some of us have seen them. One characteristic of those guns is the "twisted" cylinder. They couldn't get steel for the cylinder and were forced to use iron, but for additional strength they heated the iron rods and twisted them, leaving a sort of "barber pole" effect which was clear on the two I saw. The trigger guard is also larger than it should be, and the shape of the frame at the top of the grips is wrong. I agree that it is, if only on the odds, a repro, but would like to actually see it.
AFAIK, the highest known S&B serial number is 1234. Almost all have the "C.S." marking on the front side of the frame, one side or the other.
Jim